matching problem with custom antenna ST25R3911b

[Nathaniel Arzoine]

Hi all,

I working with 2 EVB for ST25R3911b: ST25R3911B-DISCO

the ST25R3911B-Disco Board have a 2 loop ~950nH (105 mm x 52 mm) antenna

I tried to replace the antenna for a custom antenna ( 80x48.5mm ~940nH ) very similar to the ST25R3911B-Disco Board.

The Impedance is 947nH, according to eDesignSuite.

According to AN5276 I need a network analyzer (that I don't have) to measure all the parameters of the antenna.

I tried to use the NFC Tuning Circuit, but because a don't have the exact parameters of the antenna so I tried to smart guess the rest of the parameters.

In result, the circuit just don't work at all, I connected the board to the software "ST25R3911b Discovery GUI" to measure the Antenna, and I got "out of range" message with a vector of 0.07 with 30Deg. (need to be 2.5 with 90Deg)

I don't understand how the result is so off.

There is a way to simulate the parameters of my antenna?

thank you very much

[Travis Palmer · Accepted solution]

Hello Nathaniel,

unfortunately, it is always good to confirm simulation results by measuring the smith chart.

I would recommend you to simply go to a company (or university) who owns a VNA and perform the measurements.

A common tool you could use the the miniVNA pro. It is sufficiently for measuring antennas and verify matchings.

You could also rent a VNA.

BR Travis